The soldier ant in Nethack has no special/magical abilities—and yet is the most common cause of deaths in the game, thanks to their speed and numbers. It's not the only monster with a reputation, but fights for the spot of worst creature with floating eyes, leprechauns, nymphs, gremlins, eels, krakens, mind flayers, master mind flayers, green slimes, cockatrices, chickatrices, black dragons, liches, arch-liches, rust monsters, disenchanters, iron golems, air elementals, the Gnomes With The Wand Of Death, and minotaurs to an extent. By this point, the astute reader has picked up that Nethack isn't a "game" as much as an extremely prolonged form of masochism. Ask any serious player.
You left out Vrocks. They summon other Vrocks. The summoned Vrocks then go on to summon even more Vrocks. We are sorry that "Vrock" now no longer looks like a word.
Werecreatures (wererats, werejackals, and werewolves) can infect you with lycanthropy for every attack that hits. Lycanthropy causes you to randomly polymorph into one of the same types of monster that attacked you, unless you have protection from shape changers. Wererats and werejackals have such terrible carrying capacity that you'll almost certainly by incapacitated as soon as you shift, while werewolf transformations will destroy any shirt, armor, or cloak you're wearing. Even worse, lycanthropy is nigh-impossible to cure; if you can't pray for whatever reason, you just have to hope you run across a sprig of wolfsbane or a bottle of holy water.
Nymphs don't directly hurt you, but you'll be wishing they did. They steal items, up to and including the weapon out of your hand and the armor off your back. They can also immobilize you, during which time they'll steal literally everything you have. Even worse, they'll teleport away as soon as you get ready to hit them back. If you get unlucky, it's very likely you'll end up running around the floor in your skivvies with whatever weapon you can grab off the floor, hoping you can corner her and get a good hit in to kill her before you run into whatever otherDemonic Spiders are on the floor. Leprechauns are similar but less evil, preferring instead to steal gold.
On the bright side, both Nymphs and Leprechauns tend to spawn sleeping, where you can slip by them if you don't wake them up. That won't help you, of course, when a water nymph pops out of the fountain you were dipping your longsword in.
Most quadruped enemies are Mighty Glaciers; slow, but obscenely powerful once they get close. Except NetHack tends to deemphasize the 'glacier' part, while focusing on the might. The most infamous of these is the mumak, a ferocious war elephant with a headbutt attack that does 4d12 damage. At the levels you're likely to start encountering them, a couple of those can almost certainly kill you, and they get multiple attacks per turn, at least one of which will be a headbutt.
Then, you have the exception to the rule: the leocrotta. It's not only exceptionally powerful, but it's incredibly fast.
Nethack, at least, includes one way to relieve yourselves of your most hated Demonic Spiders: A scroll of genocide can wipe out an entire species (for example, one type of lich) or an entire class of monsters (for example, all liches), depending on how blessed it is. Unfortunately, this won't work on some enemies.
Mimics, which hide in shops and disguise themselves as items. If you don't have the Genre Savvy to recognize an item that doesn't belong in the shop, or if it's a general store, then you'll end up stumbling across it and getting attacked.
Every monster in this game that aren't Goddamn Bats are Demonic Spiders. Arguably the only monster that doesn't leave you feeling sore if you aren't properly prepared would be the Grid Bugs, tiny little bugs who occasionally hit for a whole point of damage, and Lichens, which, while similar to much more deadly fungi, only has a passive that makes you stick to it and dies in a couple hits (Sometimes dropping vegan food that never rots!).
Giant eels, electric eels, and krakens only live in water, which should make them mostly harmless, right? Wrong. When you pass by them, they'll pop out and grab you. If you can't get away from them or kill them within a few turns, they'll drag you into the water and drown you.
ADOM has summoners, such as werewolf lords, which create other summoners, which in turn summon others, filling the level before you take 4 or 5 steps towards it. Due to a feature of the levelling system which strengthens monsters according to how many you have killed, this quickly becomes tiresome, and later on, lethal. Blink dogs that teleport away to multiply, jackalweres, dark elven wizards which can call spiders that are now as strong as giants... It's no wonder that one of the best pieces of advice for newbies is, "Kill breeders/summoners on sight. If you've killed too many minions, run."
The Battle Bunnies. A level filled with fast-breeding bunnies with Nasty Sharp Pointy Teeth. What's more fun? Scrolls of Vermin Control, which normally "neuter" every breeding creature on the level, go up in smoke on this level. The player's only chance is to use magic to beeline through the crowd and take down Bugs Bunny, thereby keeping the remainder from breeding. Though Bugs is not actually described as being a rabbit himself.
Ghosts can pass through walls and have an aging touch, making them this to any player race with a short lifespan. A "tension room" full of ghosts can easily surround an orc or troll in a corridor and age him to death.
Ogre magi are at least as tough and strong as normal ogres, and can cast invisibility on themselves and ice spells at the player.
Dark elven priestesses, princesses and wizards (called spider factories) are particularly nasty summoners. Not only they summon hordes of spiders, which fill the area with immobilising webs and use poison, but have themselves nasty attacks such as paralyzation and energy ray spells.
TOME has the Nazgul aka the Ringwraiths. If they hit you, you get infected by the Black Breath, which slowly drains your stats and hard-earned experience. If you hit them it destroys your weapon and gives you Black Breath, and it doesn't even hurt them if it isn't a magical weapon. Magical weapons count...but they get damaged, and it's very hard to fix them. This is in addition to the other magical attacks and summons they can do. Oh, and even if you luck out and "kill" them, they'll keep coming Back from the Dead until Sauron is killed. Permanently, as Sauron will keep coming back until the One Ring is either used or destroyed. Not a bad depiction of them, actually.
More annoying to me are RNGs, when you first meet them, in the forest. One is easy to kill. But they breed, so if they are awake you'll be facing more than one. They hit to confuse, meaning you can't aim attacks, or use any escape/healing magic. (And they occur several levels before confusion resist becomes common.) Oh, and they can move through trees, so that forest that's limiting your sightlines and probably hindering your movement? Yeah, the'll come straight through that.
TOME 4 has...any mages and ranged attackers. The systems was designed such that those enemies are always deadly. When you get to the second half of the game, add Orc Berserkers and Elite Berserkers. And the horrors.
Although the archers' threat are lessened if you're capable of doing ranged attack, they can still be a pain if you fight them in dark dungeons without good light source or spells, since they can see you when you can't see them.
Mages can do a lot of damage, and when faced with corruptors and blood mages, they are capable of inflicting you with diseases and even lower your resistance, which means that all following up attacks will deal even more damage.
Berserkers have loads of health, can rush you from a few squares away, and deal a crap ton of damage per hit.
The horrors vary wildly, some are easy, some has very nasty attacks, but the worst thing is if you face ones in the latter category that is generated with crapload of resistance.
The Slimes in Rogue, due to their tendency to replicate themselves and surround you.
A large number of enemies are brutal in the roguelikeSpelunky. While common enemies like the bats aren't too difficult to kill, some are just plain evil:
The yetis in the Ice Caves from Areas 9 to 12 are immune to your normal whip attack, somewhat durable, and can chain-throw you to death easily. Many deaths often result in these evil creatures tossing your corpse back and forth like a baseball.
The Cultists in the final areas of the game are worse, as they are not only capable of said chain-throws, they are also fast and can jump after you.
Dungeon Crawl, while not as punishing as Nethack, still has many non-unique enemies that are quite unfair. Most of them have a powerful smiting attack, i.e. a ranged attack that cannot miss and does not require line of sight (which means the player is unable to retaliate if other enemies are in the way, barring the use of one of the quite rare (and hard to cast) smiting player spells.
The earliest ones the players encounters are the orc priests, often found before level 5 of the dungeon. They have a smite attack allowing them to kill you with holy wrath from their god while standing safely behind a couple of regular orcs that shield them from anything you could cast at them. The best defense at this point is having a spell with an area of effect, which is part of the reason Mephitic Cloud (as the only low-level area spell, which has the benefit of disabling casting) is so damn unseful.
Jellies are usually not a problem for any caster, but melee fighter can severely damage their equipment trying to fight them, unless they have already found some artifacts, highly enchanted equipment, or dwarven equipment. It gets worse for ranged fighter as they heal by eating the ammo you throw at them, unless it's rocks, which don't do that much damage in general. Luckily, they are quite slow so unless you get cornered you can usually get them.
The Spider Nest is a branch that has one chance out of two of appearing instead of the Snake Pit in a game. As the name implies, it's full of spiders, and unless you are poison resistant (or even better, poison immune, which means you are playing a Gargoyle or an undead) many of those qualify as Demonic Spiders. Some have an extremely nasty poison that will bring down the toughest characters in a few turns, which can quickly burn through your stash of potions of healing. Some have an attack that confuses the character, allowing their allies to swarm you. Some throw insanely powerful orbs of destruction at you while staying on the edge of your line of sight. And worst of all, the ghost moth. It's invisible, extremely tough, very fast, it eats away you magic just by being in your field of vision (except you can't see it... because it's invisible), and has a confusion-inducing melee attack. Oh, and unlike most of the denizens of the Spider Nest, it's also poison resistant.
Almost every monster specific to the Abyss, with the exception of abominations, is a Demonic Spider. One of the worst offenders is probably the Wretched Star, which has a smiting attack that gives you bad mutations. Those mutations go away eventually as you kill monster, but it's not that easy to kill things when you're considerably slower than monsters, have -30% HP and MP as well as diminished stats, don't regenerate HP anymore, shout regularly to attract monster's attentions and have a 30% chance of failure when using scrolls...
Anything with a projectile-based weapon in Dwarf Fortress. On paper, arrows and bolts deal about as much damage as any other attack (non-players, please note that 'any other attack' is in this case a hit from a spear, and that a particularly fine kitten-bone bolt does as much damage as a basic iron spear). However, they deal 'piercing' damage, which means that any arrow that hits will more than likely deliver fatal damage to your various organs. Not only that, but they seem to completely ignore armor all together, are nearly unblockable, rarely miss or 'glance away', meaning that if that arrow has your name on it and it hits you, you'll probably end up on the ground bleeding to death. Worse yet, archers and such have virtually no attack delay, meaning that they fire off shots like they're holding a machine gun more than a piece of wood with string attached. In Adventure mode, they can also shoot at you from several screens away, meaning that your Legendary Swordmaster, who can effortlessly fight off entire crowds of foes and strike down Demons without taking a single hit, can be killed by a single wooden arrow fired by a novice archer that shouldn't even be able to see you. And then, on Fortress Mode, you can have entire armies of bowmen descend upon you, perforating your champions with dozens upon dozens of missiles the minute the idiots stagger outside to do battle. It's a frustrating experience, but then again, most Roguelikes are.
Fortunately, traps are an effective method of dealing with goblin sieges.
Editing the text files containing the damage values for bolts and arrows is another option.
Elephants used to be the most feared and revered animal in the history of DF, their brutality unmatched. One good example of this was Boatmurdered; packs of elephants would suddenly attack and eat dwarves, their enormous tusks gleaming red with blood. Such was their brutality that they were named the king of all beasts, and a undead elephant was a symbol in itself in dwarven culture and society to mean brutality, terror, death and destruction.
Appropriately enough, Dwarf Fortress also has the dreaded Giant Cave Spiders, which can immobilise you with webs before you've even seen them, completely paralyse you within a few rounds, and are entirely capable of reducing you to a soggy pile of Dwarf Chunks with their basic melee attacks.
The true demonic spiders of dwarf fortress are the demons, appropriately enough. Not because they are individually tough, not because they fly, only partly because they're stealthy and avoid traps, but mostly because some of them set everything nearby on fire AND breathe massive bursts of fire. On the positive side, you'll never get more than a few dozen on any one map, and if you defeat them all you can then amuse yourself by chucking garbage and enemies into Hell.
That was written before the 2010 update. The Hidden Fun Stuff is more difficult to conquer now: The number of demons is now either several billion or infinite - nobody seems quite certain so far - and some of them, lacking multiple body parts or vital organs and blood, are functionally immortal. Evidently the developer decided the old version was too easy.
Nothing but sheer terror when a lake or stream is nearby due to carp. They attack creatures outside of water as if they were in (a creature half the size of a dwarf mind you). Their default attack is a bite that does 1-6, compared to a dwarven punch of 1-2 damage. Coupled with the fact that dwarves are not known for their swimming habits and can be dragged underwater in the blink of an eye, or even just "dodging" into the lake. It's so bad their nickname amongst the in-crowd has been known as the "Aquatic doombringer." Best suggestion when greeted with carp at the beginning of a game: restart.
The horror is unmatched when you encounter carp on a map that can generate undead. As undead carp can leave the water, and drag screaming dwarves out of their beds to their doom in the lake...
Dwarf Fortress: we promise nothing but amphibious zombie whales that silently infiltrate your fortress.
"I think I made fish too hardcore" -Toady One. "Carp" are said to stand for "Creatures of Awful Ravenous Power."
Orcs from some of the mods can veer into this; not only are they bad-tempered, fast-moving and obscenely tough, but they turn up in swarms, right from the first winter, they are immune to automated traps, and they can kick open locked doors. The only reason forts with orcs modded in survive more than an in-game year is that they are still susceptible to the more...elaborate deathtraps from the twisted imaginations of DF players, such as drowning chambers, drop chutes, and atomsmashers.
The Crown of All Beasts now rests on the fuzzy ball of anger, hate, and claws known as the badger. A single badger is no threat, but they move in huge pack able to skillfully rip even an armored dwarf to death in seconds. Worse yet, there are also Giant Badgers; no one who has ever seen a Giant Badger has lived to tell the tale. As if one becoming enraged at the slightest provocation and effortlessly ripping a dwarf to bloody ribbons alone wasn't enough, they also come in large packs that can even swarm and kill a full armoured military squad.
Not for adventurers—you can run away from giant badgers. The real fear of the adventuring wanderer is the bogeymen, who spawn in mid-sized numbers, are VERY good at dodging, can teleport to you if you run away, and only appear in total darkness. Despite being small, they can sometimes KICK IN YOUR SKULL THROUGH A STEEL HELMET. And they don't need to be great, because they dodge enough that it is almost impossible to hit one! Oh, and some can fly. The only good thing about them is that even a single companion will stop them from spawning, but it is very possible to be attacked and killed while you go to a fortress to get a companion who won't get killed on your first quest, or while going back to civilization after all of your companions died in a quest. Not much fun, but lots of Fun.
The 2012 version features "husks", zombie-like monsters with a singular hate for all life. Certain evil biomes feature clouds that turn normal creatures into husks; once they're turned, they're nigh impossible to kill and powerful to the point of being broken. Oh, and they tend to be covered in the dust that caused them to turn, which makes more husks if you touch it. And they keep all the combat skills they had before they got husked. This thread from the forums suggests that not even DEMONS can stand toe-to-toe with a swarm, or even a small band of husks.
Giant keas are surprisingly lethal for what are basically overgrown parrots with serious kleptomania. They run on a policy of step one, go into fortress; step two, try to steal stuff; step three, kill every dwarf they come across. While they can be killed, they're still fast-moving, high-damage dwarfrippers.
And then there's Giant Sponges. Yes, sea sponges. Your dwarves' Artificial Stupidity makes them fight sponges to the death- and since sponges have no vital organs, blood or even targetable limbs, they're unkillable, so the dwarves either drown trying to reach them or get pushed to death. (Yes, they have a slam attack. Despite being immobile.)
As a corollary to all of the above, anything else can be included on the list as long as its a zombie. Zombified creatures don't feel pain, which means they will not stop attacking until destroyed. Back in the day they couldn't be destroyed at all since they didn't have vital organs; a later patch turned zombies into the first enemies in the game to have Hit Points. And yes indeed, long-time players keep modding zombified creatures in to make their game even harder.
Doom The Roguelike has Arch-Viles. They revive the dead. Normally one could leave the entry at that, but they're also much faster than the player (unless geared for maximum speed) and have an undodgeable attack that hits for 20 damage. This can be lowered by resistances, but 20 damage is a lot - for reference, the most a player can start with is 60. Oh, and they have a level dedicated to them.
There's more than just that. Pretty much everyone that was a Demonic Spider in the original keeps the status here, and some that were not that bad got horrid here, like the Mancubi, who are much harder to dodge, and getting caught by the three rockets when near a wall is usually a death sentence.
FTL: Faster Than Light has the dreaded Boarding Drone. Unlike normal boarding parties, the drone doesn't get teleported in, instead getting fired through your ship's hull (and ignoring shields), automatically breaching whatever room it hits. Since it's a robot, it can't be suffocated; you either have to knock drone control offline (which immobilizes it) or destroy it (which means the enemy will just fire another drone, punching another hole in your ship). And they have boosted health, meaning most races can't go one-on-one and expect to live. The only bright side is that a Defense Drone mk1 can shoot them down, but it doesn't mean much when they'll just keep firing until one gets through.
Ion Storms, a randomly occurring environmental effect in Nebula nodes. Running into one cuts your reactor output in half and depowers systems at random accordingly. This works on your enemy, too, but they can generally manage greater reactor output than you can. When you have to juggle power between your weapons, shields, engines, and life support, you can expect the enemy waiting for you to put some big holes in your hull before you make it out of there. Fortunately, having the Long-Range Scanners augment you can see the storms from afar and avoid them accordingly.
Crystal ships only show up near the end of That One Sidequest, but they are very capable of putting a serious crimp in said sidequest. They're armed to the teeth with weapons unlike any you've seen before: their main weapons ignore one layer of shields and don't run on ammo, while they also have bombs that lock down a room, preventing anyone from leaving or entering the room until the effect wears off. For your sake, let's hope the final quest marker is close to the entrance, or you could very well be doing that huge sidequest again.
The Binding of Isaac has several, especially in Wrath of the Lamb, and all of which can make your life miserable.
Trites from Wrath of the Lamb are literal spiders with upside-down humanoid faces and blood on their teeth. They have a fairly large amount of health, and have a very erratic and tough leaping attack like that of a Hopper, but farther and faster. The worst part is that they can be found immediately as the game starts, should you start in the Cellar.
Knights are found in The Depths and onwards, and amble around the room until you cross a cardinal direction to them in a medium range, at which point they charge you until they hit something. They can also only be hit from the back, and often hide in between thin rocks, making it extremely difficult to circle them and maybe impossible depending on your speed and shot speed. They have an upgrade in the form of Selfless Knights, which are even more aggressive and even harder to circle around.
Mask + Hearts from Wrath of the Lamb are a floating mask and disjointed human heart that act as one being. The invulnerable mask chases you around the room similar to the Knight, but faster, while the heart attempts to hide from you and shoots in the four cardinal directions rapidly if you corner it. Often the masks will chase you into a corner of the room while the hearts huddle in the opposite corner, and should you dodge all the masks, you will take damage from the hearts' shots.
Babies teleport around the room, screaming and shooting blood tears at you. Their teleports make them a tricky target to hit, and the constant confusing volleys of shots makes it very difficult to dodge. Their upgrade in Wrath of the Lamb, the Angelic Baby, is far worse as it has a triple shot that further crimps your dodging ability.
Hangers from Wrath of the Lamb are one of the worst by far. They dangle from the ceiling and slowly edge around the room, shooting triple shots whenever it locks onto you. Every time it shoots you, you lose three cents, two of which are dropped for you to possibly reclaim and one which is lost forever. Touching one of these Hangers results in incredibly fast money drain in addition to damage. To make matters worse, they are orbited by an Eternal Fly, which blocks shots that hit it and breaks free to hit you when its parent Hanger dies. And Hangers come in groups of two or three, often in rooms with turrets.
Holy Leeches, also from Wrath of the Lamb, are incredibly difficult. They are upgraded Kamikaze Leeches, which rush at you very quickly and explode, dealing large amounts of damage. They also have an Eternal Fly to block shots, and have upgraded health. They spawn in groups of five, meaning you will inevitably be cornered.